Increased global nitrous oxide emissions from streams and rivers in the Anthropocene

Emissions of nitrous oxide (N2O) from the world's river networks constitute a poorly constrained term in the global N2O budget(1,2). This N2O component was previously estimated as indirect emissions from agricultural soils(3) with large uncertainties(4-10). Here, we present an improved model representation of nitrogen and N2O processes of the land-ocean aquatic continuum(11) constrained with an ensemble of 11 data products. The model-data framework provides a quantification for how changes in nitrogen inputs (fertilizer, deposition and manure), climate and atmospheric CO2 concentration, and terrestrial processes have affected the N2O emissions from the world's streams and rivers during 1900-2016. The results show a fourfold increase of global riverine N2O emissions from 70.4 +/- 15.4 Gg N2O-N yr(-1) in 1900 to 291.3 +/- 58.6 Gg N2O-N yr(-1) in 2016, although the N2O emissions started to decline after the early 2000s. The small rivers in headwater zones (lower than fourth-order streams) contributed up to 85% of global riverine N2O emissions. Nitrogen loads on headwater streams and groundwater from human activities, primarily agricultural nitrogen applications, play an important role in the increase of global riverine N2O emissions. N2O emissions from rivers have increased globally by a factor of four between 1900 and 2016, with emissions starting to decline since the early 2000s. Most riverine N2O emissions come from smaller streams, driven primarily by the use of nitrogen fertilizers in agriculture.